The *.img, *.unl & *.gma files are interdependent so it's best they be in the same folder, either on the device (or a card) which the .unl & .gma were originally created for. If you move the map to an sd card to use it in a
different unit, it needs to be unlocked, and that means by
[Only registered and activated users can see links. Click Here To Register...]. Otherwise you would need to repurchase the map and obtain a new .unl & .gma. You can create a new .unl file for the map using the card id with jetmouse, but it'll be useless without a new .gma of course, because the original .gma is specific for the original device/card only, so it won't work with the new .unl.
Edit: Just to be clear, the map will work in
any sd inserted in the unit provided it's accompanied by it's legitimate and appropriately named .unl & .gma files. Accompanied that is, either on the sd with the map, or remaining on the unit. By legitimate, i mean those files that were created or existing when the map/unit was purchased.